The present invention relates, in general, to aircraft instrumentation and control systems and, more particularly, to apparatus for computing the gross weight and center of gravity of the aircraft by measurements of the longitudinal axis and normal axis acceleration components in conjunction with air data parameters, pressure ratios or fan speeds of the engines and the positions of the flap-slat and horizontal stabilizer aerodynamic control surfaces. The aircraft gross weight signal, in conjunction with the flap-slat configuration of the lifting surface, is used for automatic determination of essential take-off speed references such as decision speed (V.sub.1), rotation speed (V.sub.R) and safety speed (V.sub.2). The weight is also a basic parameter for the management of thrust, drag and lift during climb, cruise, letdown and holding patterns to obtain optimum fuel economy and optimum long range flight. It is also used for the determination of proper stall margin approach speeds.
An accurate measure of the center of gravity is useful when transfer of fuel between tanks is made during flight. It is generally desirable to maintain the center of gravity to be aligned with the center of aerodynamic lift on the wing since this will result in minimum load on the horizontal stabilizer and thus reduce aircraft drag. The center of gravity signal is also useful for controlling the mechanism which adjusts the elevator feel as reflected at the pilot's controls to maintain the stick force per unit of normal acceleration reasonably constant over the operating airspeed range of the aircraft.
Prior art techniques are based on an approximate knowledge of the empty weight of the aircraft and piece meal accounting of the weights and locations of the fuel and payloads that are added and subsequent totalizing of the fuel flow as it is consumed by the engines. This process is awkward since it does not lend itself to rapid updating of the weight and balance estimate and is subject to error because of initial empty weight, fuel and payload weight approximations and because of cumulative errors inherent in fuel flow rate integration and the density variaions of volumetric fuel flow measuring techniques. The present invention, on the other hand, is aimed at utilizing readily available, reliable and accurate aircraft sensors, the outputs of which are processed to generate instantaneous gross weight and balance signals which are not subject to these errors.
U.S. Pat. No. 3,691,356, titled Speed Command and Throttle Control for Aircraft, issued Sept. 12, 1972 to the present inventor and assigned to Sperry Rand Corporation, represents related prior art that discloses the automatic computation of gross weight from measures of parameters that are not dependent on estimates of empty aircraft weight or accounting of the status of fuel and payload. The technique disclosed in the prior patent, however, is only applicable to aircraft that are completely airborne. It is not capable of generating an accurate weight signal when all or part of the aircraft's weight is supported by the landing gear. The present invention differs from the previous aforementioned patent in that it utilizes a measure of the thrust generated on the aircraft by the engines and, as will be demonstrated, this additional data enables the aircraft weight to be determined very shortly after the start of the take-off run and thus is available for automatic and independent derivation of the critical take-off speed references V.sub.1, V.sub.R and V.sub.2 which are a function of the take-off gross weight of the aircraft, which references may then be used to automatically set the corresponding indices of an airspeed indicator for use by the pilot during the ground roll and take-off.